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On Saturday, new rules went into effect governing the maximum amounts of time flight crews can work, just as Mother Nature was taking dead aim at the nation's biggest airports. (Published Thursday, Jan. 9, 2014)

As Chicago and other cities saw thousands of flight delays and cancellations during the recent snowstorms and subzero weather, there was another element at work, which may become an increasing factor in America's love affair with air travel.

The clock.

On Saturday, new rules went into effect, governing the maximum amounts of time flight crews can work. It was only a tragic confluence of events, some would say with little irony a "perfect storm", that the long-scheduled rules kicked in even as Mother Nature was taking dead aim at the nation's biggest airports.

Under the new rules, pilots' lives are governed by at least eight different clocks, ticking away on criteria like total duty time, the number of segments to be flown, actual time in the air, and the hour at which the work day began.

Flight crews can now only work a 9 to 14 hour day, versus 16 under the old rules. Only eight or nine of those hours, depending on the circumstances, can be actual flight time, and pilots must get a minimum of 10 hours off between shifts, including at least eight hours of uninterrupted sleep.

Throw in delays, and it could mean the flight never leaves, even if the weather clears, the passengers are on board, and the aircraft is fueled and ready to go. For example, if the pilot reports for work, but faces a three hour weather delay, crucial decisions have to be made whether he or she can complete all scheduled legs before time expires on the work day.

If all of those legs cannot be completed, and there are no relief pilots in sight along the way, the entire route might have to be cancelled.

The changes are certainly no surprise. The rules were passed two years ago, with this week's start date published long in advance. Pilots and safety agencies had long advocated new work rules to combat crew fatigue. Most point to the tragic 2009 crash of a Colgan Air regional jet in upstate New York, where fatigue was considered a contributing factor, as the final straw which forced the issue.

Neither the airlines nor pilots' unions would speculate over how many of this week's cancellations were directly attributable to the new scheduling rules. All agree that the severe weather was responsible for the lion's share of the problems, and that the flurry of delays and cancellations would have happened under the old rules as well.